Gas turbine engines typically include a turbine module that is arranged to drive a compressor module via one or more interconnecting shafts. For example, in a ‘three-shaft’ gas turbine engine, a propulsive fan is driven by a low pressure turbine via the low pressure shaft. The low pressure shaft comprises two shafts, specifically, a low pressure turbine shaft and a low pressure compressor shaft which are made of different materials. The low pressure turbine shaft and the low pressure compressor shaft are joined coaxially and end to end by a helical spline joint.
The spline region of the low pressure turbine shaft operates at a relatively low temperature (approximately 150 Celsius), but may require a challenging combination of mechanical properties, for example, high torque carrying capability, high ultimate tensile strength, high 0.2% proof stress, and good fatigue strength. Consequently, the spline region of the low pressure turbine shaft is usually made from an alloy such as AerMet® 100. However, this alloy is not suitable for use at elevated temperatures (for example, above 400 Celsius) for extended periods of time because of over-aging of the carbide structures, which may significantly reduce the yield and tensile strengths and has an adverse effect on the creep resistance.
The rear part of the low pressure turbine shaft is not subject to the torque loads experienced by the spline, but may reach temperatures of up to 450 Celsius for extended periods. The rear part of the low pressure turbine shaft is therefore usually made from an alloy such as Super-CMV which is welded to the spline region of the low pressure turbine shaft. This alloy does not have the torque carrying capability or the fatigue strength to be used in the low temperature spline region of the shaft, but the microstructure is relatively stable up to 450 Celsius giving the alloy good thermal stability and creep capability.
It should be appreciated from the preceding paragraphs that the manufacture of the low pressure turbine shaft is relatively complex as it is fabricated from sections made from two different materials. Furthermore, the presence of a welded joint increases cost and manufacturing time.